1. Field of the Invention
The present invention relates to equipment and methods for manufacturing an optical fiber.
2. Description of the Background Art
It is difficult to form the core portion and the cladding portion in a perfectly circular and concentric shape in manufacturing an optical fiber. It is often the case that the core portion and the cladding portion become slightly ellipsoidal or distorted such that the cross-sectional refractive index profile of the optical fiber is not axial symmetry. As a result, the group velocities of mutually orthogonal two polarization modes that propagate in the optical fiber tend to differ from each other, and accordingly the polarization mode dispersion (PMD) increases. It is known that for manufacturing an optical fiber from an optical fiber preform, the drawing process is performed while affording twists to the optical fiber so that the PMD may be reduced.
FIGS. 5A and 5B are drawings illustrated in Japanese Patent Application Publication No. H9-243833: FIG. 5A is a conceptional schematic diagram and FIG. 5B is a perspective view. In this method, the twists are afforded to an optical fiber 1 by using an oscillating guide roller 2. In the oscillating guide roller 2, the axial center X, about which it turns, oscillates alternately in the range of +θ and −θ, and as the arrow “a” indicates, the roller surface 2a sways between the position defined by the solid line and the position defined by the dotted line. Accordingly, an optical fiber 1 which has passed a guide bar 3 rolls on the top of the swaying roller surface 2a so that twists are afforded thereto in an alternate twisting direction.
The oscillating mechanism 10 has a pinion gear 12 arranged to turn on a base 11, and the oscillating guide roller 2 is supported by means of a supporting shaft 14 so as to be mounted to a roller support body 13 that is fixed on the pinion gear 12. Also, one end of a drive arm 16 is connected with a pin 16a to a turning plate 15 driven by a motor on the base 11 and the other end is connected to a slide member 19 arranged on a movable member 17. The movable member 17 has a rack gear 18 which is meshed with the pinion gear 12 so as to afford a turning drive to the pinion gear 12, and a guide part 17a helps the movable member 17 to move to and fro in the direction indicated by an arrow “b” along the guide groove provided on the base 11. Also, a guide protrusion 19a guides a slide member 19 to slide in the direction indicated by an arrow “c”.
When the turning plate 15 is driven to turn at a constant speed, the pin 16a of the drive arm 16 moves in a circle such that the arm 16 moves to and fro. As a result of the movement of the drive arm 16, the slide member 19 moves in the direction of the arrow “c” on the guide protrusion 19a while the movable member 17 performs a linear movement in the direction of the arrow “b”. The pinion gear 12, which meshes with a rack gear 18, is caused to perform an oscillating rotational movement in the direction of an arrow “d” by the linear movement of the movable member 17. The oscillating rotational movement “d” of the pinion gear 12 causes the oscillating guide roller 2 to oscillate in the oscillating range “a”.
FIGS. 6A and 6B are drawings which illustrate the method described in Japanese Patent Application Publication No. 2002-226229: FIG. 6A is a side view and FIG. 6B is a front view. In this method, twists are afforded to an optical fiber 1 by reciprocating rollers 21 and 22.
The reciprocating rollers 21 and 22 are supported in parallel by a base frame 25, and a first guide roller 23 arranged above the roller 21 and a second guide roller 24 arranged below the roller 22 push the optical fiber 1 onto the surface of the reciprocating rollers 21, 22. The surfaces of the reciprocating rollers 21, 22 have a coefficient of friction which is sufficient for the optical fiber 1 to roll without sliding thereon. The reciprocating rollers 21, 22 are caused to turn by the running of the optical fiber 1, while the reciprocating rollers 21 and 22 are moved to and fro in mutually opposite directions at a given period. Thus, the optical fiber 1 rolls on the surface of the reciprocating rollers 21 and 22, so that twists are afforded to the optical fiber 1. The number of the reciprocating rollers provided in such case is supposed to be one or more than two.
These known mechanisms are complicated and the relevant equipment is inevitably large-sized. Also, the driving direction of the rollers is reversed so that the direction of twist in the optical fiber may be reversed. And, at the time of such reversion, the twisting operation is interrupted temporarily. Therefore, the twisting speed is slow, and the number of twisting times becomes less, resulting in increase of variation in the PMD characteristics.